I'm setting up a DC battery bank

I have 6 X 170ah 12v AGM powersafe batteries

A Victron skylla 50amp/24v battery charger. And a 24v inverter that I'm yet to wire in

I wired 3 sets of 2 batteries together in series to make 24v, then I linked each series of batteries together in parallel to two big bus bars (equal length cables). I used 35mm2 cable. The battery charger is also connected to the bus bar

forgive my (not so) technical drawing

wiring diagram

I have set the battery charger to the correct voltage listed in the battery manufacturers data sheet (28v float at current temps).

The problem is the batteries don't seem to be charging equally. When I turn the charger on, the first battery in each series reads 14.5v, the second reads 13.5v.

Have I done something wrong? I did a lot of research before starting this project, but apparently not enough.

All help appreciated thank you

  • \$\begingroup\$ Are the batteries new or used? Have you charged them individually before connecting them in series to equalize their charge? \$\endgroup\$
    – StarCat
    Commented Nov 9, 2022 at 20:25
  • \$\begingroup\$ I got them second hand , made in 2021, I did charge them individually first , the manual said they'd need a reconditioning charge after being stored so I put them on my CTEK charger , on AMG recondition settings. \$\endgroup\$ Commented Nov 9, 2022 at 20:30
  • \$\begingroup\$ @StarCat could unevenly charged batteries cause this issue ? It is possible they weren't exactly the same , the charger I used is a 5 amp charger, very small on a 170ah battery \$\endgroup\$ Commented Nov 9, 2022 at 20:45
  • \$\begingroup\$ There should be no reason why the first battery in each series should read 1V higher than the second of each series. What happens if you switch first and second batteries? \$\endgroup\$
    – PStechPaul
    Commented Nov 9, 2022 at 23:31
  • \$\begingroup\$ Are you measuring the voltage directly at the battery terminals? \$\endgroup\$
    – RoyC
    Commented Nov 9, 2022 at 23:35

1 Answer 1


To make used lead-acid batteries work together, they really need to have historically been together in the same pack.

If you just jumble 6 random batteries together that have different service histories, the pack will be degraded.

You might try carefully measuring their charge/discharge performance (the way battery builders do that with 18650 lithiums to build packs which are a matched set).

Totally separate from that, even if you have a set of 6 that has been stored awhile, they may be at different states of charge. Which is no big deal; charge them back up to full. Preferably as individual 12-volt batteries prior to assembling them into a pack.

But if you do assemble them into a pack, immediately start doing charge-discharge cycles and the problem will sort itself out after awhile. Why? Because lead-acid batteries are less efficient at higher states of charge. So if one cell is at 95% and the next cell is at 60%, the 95% cell will be charging less efficiently allowing the 60% cell to eventually catch up.

However you don't want to charge cells beyond 100% as this will result in gassing of hydrogen and oxygen, and consumption of water in the electrolyte. That still happens on sealed batteries; it's just that you can't replace the water LOL. Sealed batteries aren't magic, they just require more precise charge controllers.


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